U.S. patent number 6,867,834 [Application Number 10/088,357] was granted by the patent office on 2005-03-15 for optical compensator for lcd including stacked films of only one o-plate, one a-plate and two negative c-plate retarders.
This patent grant is currently assigned to Merck Patent GmbH. Invention is credited to David Coates, Tara Cutler, Stephan Derow, Peter Le Masurier, Owain Llyr Parri, Mark Verrall.
United States Patent |
6,867,834 |
Coates , et al. |
March 15, 2005 |
Optical compensator for LCD including stacked films of only one
O-plate, one A-plate and two negative C-plate retarders
Abstract
An optical compensator for liquid crystal displays comprising at
least one O plate retarder, at least one low tilt A plate retarder,
and at least one negative C plate retarder is described. Also
described are liquid crystal displays comprising such a
compensator.
Inventors: |
Coates; David (Dorset,
GB), Cutler; Tara (Dorset, GB), Parri;
Owain Llyr (Dorset, GB), Verrall; Mark (Dorset,
GB), Le Masurier; Peter (Dorset, GB),
Derow; Stephan (Muehltal, DE) |
Assignee: |
Merck Patent GmbH (Darmstadt,
DE)
|
Family
ID: |
8238972 |
Appl.
No.: |
10/088,357 |
Filed: |
March 18, 2002 |
PCT
Filed: |
September 13, 2000 |
PCT No.: |
PCT/EP00/08932 |
371(c)(1),(2),(4) Date: |
March 18, 2002 |
PCT
Pub. No.: |
WO01/20392 |
PCT
Pub. Date: |
March 22, 2001 |
Foreign Application Priority Data
|
|
|
|
|
Sep 16, 1999 [EP] |
|
|
99117980 |
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Current U.S.
Class: |
349/119; 349/117;
349/120 |
Current CPC
Class: |
G02F
1/133632 (20130101); G02F 2413/105 (20130101); G02F
2413/15 (20130101); G02F 2202/40 (20130101); G02F
1/133634 (20130101); G02F 2413/04 (20130101); C09K
2219/03 (20130101) |
Current International
Class: |
G02F
1/13 (20060101); G02F 001/1335 () |
Field of
Search: |
;349/117,119,120 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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|
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0576342 |
|
Dec 1993 |
|
EP |
|
0 838 713 |
|
Apr 1998 |
|
EP |
|
WO 96 10772 |
|
Apr 1996 |
|
WO |
|
WO 97 44703 |
|
Nov 1997 |
|
WO |
|
Primary Examiner: Chowdhury; Tarifur R.
Attorney, Agent or Firm: Millen, White, Zelano, Branigan,
P.C.
Claims
What is claimed is:
1. Optical compensator for liquid crystal displays comprising,
stacked films or layers of: only one O plate retarder, only one low
tilt A plate retarder having an average tilt angle,
.theta..sub.ave, of from 1 to 10.degree., and only two negative C
plate retarders.
2. Optical compensator according to claim 1, wherein the average
tilt angle, .theta..sub.ave, in said O plate retarder is from 2 to
88.degree..
3. Optical compensator according to claim 1, wherein the average
tilt angle, .theta..sub.ave, in said low tilt A plate retarder is
from 2 to 6.degree..
4. Optical compensator according to claim 1, wherein the tilt angle
in said O plate retarder varies monotonously in a direction
perpendicular to the plane of the film from a minimum value
.theta..sub.min at one surface of the film to a maximum value
.theta..sub.max at the opposite surface of the film.
5. Optical compensator according to claim 1, wherein the thickness
of said O plate and/or low tilt A plate is from 0.1 to 10
.mu.m.
6. Optical compensator according to claim 1, wherein the optical
retardation of said O plate is from 6 to 300 nm.
7. Optical compensator according to claim 1, wherein the optical
retardation of said low tile A plate is from 12 to 575 nm.
8. Optical compensator according to claim 1, wherein the O plate
comprises a linear or crosslinked polymerized liquid crystalline
material with a tilted or splayed structure.
9. Optical compensator according to claim 1, wherein the low tilt A
plate comprises a linear or crosslinked polymerized liquid
crystalline material with a slightly titled structure.
10. Optical compensator according to claim 1, wherein at least one
of the C plates is a negatively birefringent polymer film.
11. Optical compensator according claim 10, wherein said polymer
film is a negatively birefringent TAC or DAC film.
12. Optical compensator according to claim 1, wherein at least one
C plate comprises a linear or crosslinked polymerized chiral liquid
crystalline material with a helically twisted structure.
13. Optical compensator according to claim 12, wherein the helical
pitch of the chiral liquid crystalline material in said C plate is
less than 250 nm.
14. A liquid crystal display device comprising the following
elements a liquid crystal cell formed by two transparent substrates
having surfaces which oppose each other, an electrode layer
provided on the inside of at least one of said two transparent
substrates and optionally superposed with an alignment layer, and a
liquid crystal medium which is present between the two transparent
substrates, a polarizer arranged outside said transparent
substrates, or a pair of polarizers sandwiching said substrates,
and at least one optical compensator according to claim 1 being
situated between the liquid crystal cell and at least one of said
polarizers,
it being possible for the above elements to be separated, stacked,
mounted on top of each other, coated on top of each other or
connected by means of adhesive layers.
15. A liquid crystal display device according to claim 14, which is
a TN, HTN or STN display.
16. A liquid crystal display device according to claim 14, wherein,
in the optical compensator, the optical axis of the O plate and the
low tilt A plate are oriented at right angles to each other.
17. Optical compensator according to claim 1, wherein the average
tilt angle, .theta..sub.ave, in said O plate retarder is from 30 to
60.degree..
18. Optical compensator according to claim 1, wherein the thickness
of said O plate and/or low tilt A plate is from 0.2 to 5 .mu.m.
Description
FIELD OF THE INVENTION
The invention relates to an optical compensator for liquid crystal
displays and to a liquid crystal display comprising such a
compensator.
BACKGROUND AND PRIOR ART
Optical compensators are used to improve the optical properties of
liquid crystal displays (LCD), such as the contrast ratio and the
grey scale representation at large viewing angles. For example in
uncompensated displays of the TN or STN type at large viewing
angles often a change of the grey levels and even grey scale
inversion, as well as a loss of contrast and undesired changes of
the colour gamut are observed.
An overview of the LCD technology and the principles and methods of
optical compensation of LCDs is given in U.S. Pat. No. 5,619,352,
the entire disclosure of which is incorporated into this
application by way of reference.
As described in U.S. Pat. No. 5,619,352, to improve the contrast of
a display at wide viewing angles a negatively birefringent C-plate
compensator can be used, however, such a compensator does not
improve the greyscale representation of the display. On the other
hand, to suppress or even eliminate grey scale inversion and
improve the grey scale stability U.S. Pat. No. 5,619,352 suggests
to use a birefringent O-plate compensator. An O-plate compensator
as described in U.S. Pat. No. 5,619,352 includes an O-plate, and
may additionally include one or more A-plates and/or negative
C-plates.
The terms `O-plate`, `A-plate` and `C-plate` as used in U.S. Pat.
No. 5,619,352 and throughout this invention have the following
meanings. An `O-plate` is an optical retarder utilizing a layer of
a positively birefringent (e.g. liquid crystal) material with its
principal optical axis oriented at an oblique angle with respect to
the plane of the layer. An `A-plate` is an optical retarder
utilizing a layer of uniaxially birefringent material with its
extraordinary axis oriented parallel to the plane of the layer, and
its ordinary axis (also called `a-axis`) oriented perpendicular to
the plane of the layer, i.e. parallel to the direction of normally
incident light. A `C-plate` is an optical retarder utilizing a
layer of a uniaxially birefringent material with its extraordinary
axis (also called `c-axis`) perpendicular to the plane of the
layer, i.e. parallel to the direction of normally incident
light.
As an O-plate retarder for example an optical retardation film
(hereinafter abbreviated as ORF) comprising a layer of a liquid
crystal or mesogenic material with tilted or splayed structure can
be used.
As an A-plate retarder for example a uniaxially stretched polymer
film, like for example a stretched polyvinylalcohol (PVA) or
polycarbonate (PC) film, can be used. Alternatively, an A-plate
retarder may comprise for example a layer of a positively
birefringent liquid crystal or mesogenic material with planar
orientation.
As a negatively birefringent C-plate retarder for example a
uniaxially compressed polymer film can be used. Alternatively, a
negatively birefringent C-plate may comprise for example a layer of
a liquid crystal or mesogenic material with a planar orientation
and a negative birefringence. Typical examples of negatively
birefringent liquid crystal materials are various kinds of discotic
liquid crystal compounds.
In addition to U.S. Pat. No. 5,619,352, optical compensators
comprising one or more O plates are described in prior art in WO
97/44409, WO 97/44702, WO 97/44703 and WO 98/12584, the entire
disclosure of which is incorporated into this application by way of
reference. WO 97/44703 and WO 98/12584 further suggest to use O
plates in combination with a A plate. WO 97/44703 reports that the
use of a compensator comprising a O plate in combination with a A
plate, wherein the principal optical axes of both ORFs are oriented
at right angles to each other, allows particularly good
compensation of a TN-LCD, as it simultaneously reduces the angle
dependence of the contrast and the grey scale inversion in the
display.
However, when using compensators as described in the above
mentioned prior art in combination with liquid crystal displays,
especially TN or STN-displays, the improvements of the optical
properties of the display, like contrast at wide viewing angles,
grey scale level stability, and suppression of grey scale
inversion, are still far from sufficient for most applications.
Therefore, it is desirable to have available improved optical
compensators to further improve the optical performance of
LCDs.
Definition of Terms
In connection with optical polarization, compensation and
retardation layers, films or plates as described in the present
application, the following definitions of terms as used throughout
this application are given.
For the sake of simplicity, the term `liquid crystal material` is
used hereinafter for both liquid crystal materials and mesogenic
materials, and the term `mesogen` is used for the mesogenic groups
of the material.
The terms `tilted structure` or `tilted orientation` means that the
optical axis of the film is tilted at an angle .theta. between 0
and 90 degrees relative to the film plane.
The term `splayed structure` or `splayed orientation` means a
tilted orientation as defined above, wherein the tilt angle
additionally varies monotonuously in the range from 0 to 90.degree.
preferably from a minimum to a maximum value, in a direction
perpendicular to the film plane.
The term `low tilt structure` or `low tilt orientation` means that
the optical axis of the film is slightly tilted or splayed as
described above, with the average tilt angle throughout the film
being between 1 and 10.degree..
The term `planar structure` or `planar orientation` means that the
optical axis of the film is substantially parallel to the film
plane. This definition also includes films wherein the optical axis
is slightly tilted relative to the film plane, with an average tilt
angle throughout the film of up to 1.degree., and which exhibit the
same optical properties as a film wherein the optical axis is
exactly parallel, i.e. with zero tilt, to the film plane.
The term `helically twisted structure` relates to a film comprising
one or more layers of liquid crystal material wherein the mesogens
are oriented with their main molecular axis in a preferred
direction within molecular sublayers, with this preferred
orientation direction in different sublayers being twisted around a
helix axis that is substantially perpendicular to the film plane,
i.e. substantially parallel to the film normal. This definition
also includes orientations where the helix axis is tilted at an
angle of up to 2.degree. relative to the film normal.
The term `homeotropic structure` or `homeotropic orientation` means
that the optical axis of the film is substantially perpendicular to
the film plane, i.e. substantially parallel to the film normal.
This definition also includes films wherein the optical axis is
slightly tilted at an angle of up to 2.degree. relative to the film
normal, and which exhibit the same optical properties as a film
wherein the optical axis is exactly parallel, i.e. with no tilt, to
the film normal.
For sake of simplicity, an optical film with a tilted, splayed, low
tilted, planar, twisted and homeotropic orientation or structure is
hereinafter being shortly referred to as `tilted film`, `splayed
film`, `low tilt film`, `planar film`, `twisted film` and
`homeotropic film`, respectively.
Throughout this invention, both a tilted and a splayed film will
also be referred to as `O plate`. A planar film will also be
referred to as `A plate` or `planar A plate`. A low tilt film will
also be referred to as `low tilt A plate`. A twisted film will also
be referred to as `twisted A plate`.
The average tilt angle .theta..sub.ave is defined as follows
##EQU1##
wherein .theta.'(d') is the local tilt angle at the thickness d'
within the film, and d is the total thickness of the film.
The tilt angle of a splayed film hereinafter is given as the
average tilt angle .theta..sub.ave, unless stated otherwise.
A `tilted film` or `O plate` with tilted or splayed structure
according to the present invention exhibits an average tilt angle
of at least 10 degrees, whereas a tilted or splayed film with an
average tilt angle of less than 10 degrees will be referred to as
`low tilt film` or `low tilt A plate`.
In tilted, planar and homeotropic optical films comprising
uniaxially positive birefringent liquid crystal material with
uniform orientation, the optical axis of the film as referred to
throughout this invention is given by the orientation direction of
the main molecular axes of the mesogens of the liquid crystal
material.
In a splayed film comprising uniaxially positive birefringent
liquid crystal material with uniform orientation, the optical axis
of the film as referred to throughout this invention is given by
the projection of the orientation direction of the main molecular
axes of the mesogens onto the surface of the film.
The term `film` as used in this application includes
self-supporting, i.e. free-standing, films that show more or less
pronounced mechanical stability and flexibility, as well as
coatings or layers on a supporting substrate or between two
substrates.
The term `liquid crystal or mesogenic material` or `liquid crystal
or mesogenic compound` should denote materials or compounds
comprising one or more rod-shaped, board-shaped or disk-shaped
mesogenic groups, i.e. groups with the ability to induce liquid
crystal phase behaviour. The compounds or materials comprising
mesogenic groups do not necessarily have to exhibit a liquid
crystal phase themselves. It is also possible that they show liquid
crystal phase behaviour only in mixtures with other compounds, or
when the mesogenic compounds or materials, or the mixtures thereof,
are polymerized.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1a and 1b depict uncompensated prior art TN-LCD devices.
FIGS. 2a and 2b depict compensated TN-LCD devices with compensators
according to a preferred embodiment of the present invention.
FIG. 3 depicts a compensated TN-LCD device with a compensator
according to a preferred embodiment of the present invention.
FIG. 4a is an isocontrast plot of an uncompensated prior art TN-LCD
device according to comparison example A.
FIGS. 4b and 4c are grey level diagrams of an uncompensated prior
art TN-LCD device according to comparison example A in horizontal
(4b) and vertical (4c) viewing planes.
FIG. 5a is an isocontrast plot of an uncompensated prior art TN-LCD
device according to comparison example B.
FIGS. 5b and 5c are grey level diagrams of a conventional
uncompensated TN-LCD device according to comparison example B in
horizontal (5b) and vertical (5c) viewing planes.
FIGS. 6a, 7a and 8a are isocontrast plots of an inventive
compensated TN-LCD device according to example 1-3,
respectively.
FIGS. 6b, 6c, 7b, 7c, 8b and 8c are grey level diagrams of an
inventive compensated TN-LCD device according to example 1-3,
respectively, in horizontal (b) and vertical (c) viewing
planes.
SUMMARY OF THE INVENTION
One aim of the present invention is to provide an optical
compensator which has improved performance for compensation of
LCDs, is easy to manufacture, in particularly for mass production,
and does not have the drawbacks of prior art compensators as
described above. Other aims of the present invention are
immediately evident to the person skilled in the art from the
following detailed description.
The inventors of the present invention have found that the above
drawbacks can be overcome, and an optical compensator with superior
performance for compensation of the optical properties of liquid
crystal displays can be obtained by using a combination of at least
one O plate retarder, at least one low tilt A plate retarder and at
least one negative C plate retarder.
When using an optical compensator according to the present
invention in an LCD, the contrast at large viewing angles and the
grey level representation of the display are considerably improved,
and grey scale inversion is suppressed. In case of coloured
displays, the colour stability can be improved and changes of the
colour gamut can be suppressed. Furthermore, a compensator
according to the present invention is particularly suitable for
mass production.
One object of the present invention is an optical compensator for
liquid crystal displays, characterized in that it comprises
at least one O plate retarder,
at least one low tilt A plate retarder,
at least one negative C plate retarder.
Another object of the invention is a liquid crystal display device
comprising the following elements
a liquid crystal cell formed by two transparent substrates having
surfaces which oppose each other, an electrode layer provided on
the inside of at least one of said two transparent substrates and
optionally superposed with an alignment layer, and a liquid crystal
medium which is present between the two transparent substrates,
a polarizer arranged outside said transparent substrates, or a pair
of polarizers sandwiching said substrates, and
at least one optical compensator according to the present
invention, being situated between the liquid crystal cell and at
least one of said polarizers,
it being possible for the above elements to be separated, stacked,
mounted on top of each other or connected by means of adhesive
layers in any combination of these means of assembly.
DETAILED DESCRIPTION OF THE INVENTION
Preferred embodiments of the present invention relate to an optical
compensator comprising at least one O plate and at least one low
tilt A plate as described above, wherein
the average tilt angle .theta.ave in the O plate is from 2 to
88.degree., preferably from 30 to 60.degree.,
the tilt angle .theta. in the O plate varies monotonuously in a
direction perpendicular to the plane of the film,
the tilt angle .theta. in the O plate varies from a minimum value
.theta..sub.min at one surface of the film to a maximum value
.theta..sub.max at the opposite surface of the film,
.theta..sub.min in the O plate is from 0 to 80.degree., preferably
from 1 to 20.degree.,
.theta..sub.max in the O plate is from 10 to 90.degree., preferably
from 40 to 90.degree.,
the average tilt angle .theta..sub.ave ' in the low tilt A plate is
from 1 to 10.degree., in particular from 2 to 6.degree., very
preferably from 3.5 to 4.5.degree.,
the tilt angle .theta.' in the low tilt A plate varies
monotonuously in a direction perpendicular to the plane of the
film,
the tilt angle .theta.' in the low tilt A plate varies from a
minimum value .theta..sub.min ' at one surface of the film to a
maximum value .theta..sub.max ' at the opposite surface of the
film,
.theta..sub.min ' in the low tilt A plate is from 0 to 10.degree.,
preferably from 0.5 to 3.degree.,
.theta..sub.max ' in the low tilt A plate is from 1 to 10.degree.,
preferably from 3 to 8.degree.,
the thickness d of the O plate is from 0.1 to 10 .mu.m, in
particular from 0.2 to 5 .mu.m, very preferably from 0.3 to 3
.mu.m,
the thickness d' of the low tilt A plate is from 0.1 to 10 .mu.m,
in particular from 0.2 to 5 .mu.m, very preferably from 0.3 to 3
.mu.m,
the optical retardation d.DELTA.n of the O plate is from 6 to 300
nm, in particular from 10 to 200 nm, very preferably from 20 to 120
nm,
the optical retardation d'.DELTA.n' of the low tilt A plate is from
12 to 575 nm, in particular from 20 to 300 nm, very preferably from
30 to 200 nm,
the O plate and/or the low tilt A plate comprise a linear or
crosslinked liquid crystalline polymer,
the negative C plate comprises a chiral linear or crosslinked
liquid crystalline polymer,
the negative C plate is a bireferingent polymer film,
the negative C plate is a birefringent triacetylcellulose (TAC) or
diacetylcellulose (DAC) film.
Further preferred embodiments of the present invention relate to an
optical compensator comprising
one O plate, one low tilt A plate and one negative C plate,
especially preferably wherein the negative C plate is situated
between the O plate and the low tilt A plate,
one O plate, one low tit A plate and two negative C plates,
one O plate and one low tilt A plate, at least one of which is
provided on a negatively birefringent substrate that serves as
negative C plate.
A further preferred embodiment of the present invention relates to
a liquid crystal display comprising a liquid crystal cell, a pair
of polarizers sandwiching the cell, and one inventive compensator
as described above and below located on each side of the liquid
crystal (LC) cell between the cell and the polarizer.
Especially preferred are displays wherein
the LC cell is a twisted nematic or supertwisted nematic cell,
the optical axis of the O plate and the low tilt A plate are
oriented at right angles with each other,
the O plate is facing the polarizer and the low tilt A plate is
facing the LC cell,
in case the O plate is facing the polarizer, its optical axis is
parallel to the optical axis of the liquid crystal medium at the
nearest surface of the liquid crystal cell,
in case the O plate is facing the LC cell, its optical axis is at
right angles to the optical axis of the liquid crystal medium at
the nearest surface of the liquid crystal cell,
the O plate is situated with its low tilt surface facing the
polariser,
Especially preferred compensator stacks for inventive displays
according to the preferred embodiments as described above are shown
in table 1. Therein, LC denotes a liquid crystal cell, O denotes a
tilted or splayed O plate, A denotes a low tilt A plate, and -C
denotes a negative C plate. For the case where the O plate is a
splayed O plate, the arrow is denoting the preferred direction of
increasing tilt angle.
For sake of simplicity, the polarizers are omitted in table 1. A
display for practical applications will, however, further comprise
a pair of polarizers sandwiching the stack as shown in table 1.
In the stack formats as shown in table 1 the single retarder
components are arranged symmetrically, therefore incoming light may
enter the stack from either side.
TABLE 1 Preferred compensator stacks in inventive displays [A]
.rarw.O -C A LC A -C O.fwdarw. [B] A -C O.fwdarw. LC .rarw.O -C A
[C] -C A .rarw.O -C LC -C O.fwdarw. A -C [D] -C O.fwdarw. A -C LC
-C A .rarw.O -C [E] -C A -C O.fwdarw. LC .rarw.O -C A -C [F]
.rarw.O -C A -C LC -C A -C O.fwdarw. [G] A -C .rarw.O -C LC -C
O.fwdarw. -C A [H] -C O.fwdarw. -C A LC A -C .rarw.O -C [I] A -C -C
O.fwdarw. LC .rarw.O -C -C A [J] .rarw.O -C -C A LC A -C -C
O.fwdarw.
Particularly preferred are compensator stacks of type [H] as shown
in table 1, wherein the absolute retardation values of the O plate
and the A plate are approximately the same.
Further preferred are stacks of type [H] as shown in table 1,
wherein the optical axis of the O plate and the A plate are
oriented at right angles with each other, and oriented at an angle
of from 1 to 15.degree., preferably from 5 to 10.degree., relative
to the polarization direction of the polarizer and/or relative to
the optical axis of the liquid crystal medium at the nearest
surface of the liquid crystal cell, respectively.
The inventive optical compensators can be used for compensation of
conventional displays, in particular those of the twisted nematic
or super twisted nematic mode, such as TN, HTN, STN or AMD-TN
displays, in displays of the IPS (in plane switching) mode, which
are also known as `super TFT` displays, in displays of the DAP
(deformation of aligned phases) or VA (vertically aligned) mode,
like e.g. ECB (electrically controlled birefringence), CSH (colour
super homeotropic), VAN or VAC (vertically aligned nematic or
cholesteric) displays, in displays of the bend mode or hybrid type
displays, like e.g. OCB (optically compensated bend cell or
optically compensated birefringence), R-OCB (reflective OCB), HAN
(hybrid aligned nematic) or .pi.-cell displays.
Especially preferably the compensators are used for compensation of
TN, HTN and STN displays.
In the following, the invention will exemplarily be described in
detail for compensation of a TN display.
FIG. 1a depicts an uncompensated standard type TN display device in
its off-state, i.e. when novoltage is applied, comprising a TN cell
1 with a liquid crystal layer in the twisted nematic state
sandwiched between two transparent electrodes (which are not shown
here), and a pair of linear polarizers 2,2'. The twisted nematic
orientation of the liquid crystal layer is schematically depicted
by the mesogens 1a. The dashed lines 1b and 1c represent the
orientation direction of the mesogens 1a that are adjacent to the
cell walls of the TN cell 1.
In the display device shown in FIG. 1a, the polarization axes of
the linear polarizers 2,2' are oriented at right angles to the
optical axis 1b,1c of the liquid crystal medium at the nearest
surface of the liquid crystal cell 1, respectively. This
orientation of the polarizers relative to the TN cell is
hereinafter also generally referred to as `E mode`.
FIG. 1b depicts an uncompensated standard type TN display device
like that of FIG. 1a, but wherein the polarization axes of the
linear polarizers 2,2' are oriented parallel to the optical axis
1b,1c of the liquid crystal medium at the nearest surface of the
liquid crystal cell 1, respectively. This orientation of the
polarizers relative to the TN cell is hereinafter also generally
referred to as `O mode`.
FIG. 2a, b schematically depict compensated TN-LCD devices
according to preferred embodiments of the present invention in the
off-state, with FIG. 2a showing a device in the O mode and FIG. 2b
showing a device in the E mode, as explained above.
The devices consist of a TN cell 1 with a liquid crystal layer in a
twisted nematic state sandwiched between two transparent electrodes
(which are not shown here), a pair of linear polarizers 2,2' and
two compensators, each compensator consisting of a splayed O plate
3,3' a low tilt A plate 4,4', and two negative C plates 5,5',5" and
5'" on each side of the TN cell 1. The stack formats of the optical
components in FIG. 2a, b correspond to type [H] of table 1
above.
In the devices examplarily shown in FIG. 2a, b each of the O plates
3,3' and the low tilt A plates 4,4' are provided directly on the
negative C plates 5,5',5",5'" which serve as substrates for the O
and A plates.
The stacks of optical components in the devices shown in FIG. 1 and
FIG. 2 are symmetrical, hence incoming light may enter the device
from either side.
The C plates 3,3' consist, as an example, of a layer of polymerized
liquid crystal material with splayed structure. The splayed
structure is schematically depicted by the mesogens 3a and 3a'
which are oriented with their main molecular axis tilted at an
angle .theta. relative to the plane of the layer, wherein the tilt
angle .theta. increases in a direction normal to the film, starting
with a minimum value .theta..sub.min on the side of the O plate
3,3' facing the TN cell 1.
The dashed lines 3b and 3b' represent the projection of the
orientation directions of the mesogens 3a and 3a', respectively, in
different regions of the O plates 3,3' onto the surfaces of the
respective O plates 3,3'. The dashed lines 3b,3b' are also
identical with the principal optical axis of the respective O
plates 3,3'. In the devices shown in FIG. 2a, b, the principal
optical axes of the O plates 3,3' are oriented parallel to the
polarization direction of the respective adjacent linear polarizer
2,2', and parallel to the respective adjacent orientation direction
1b,1c of the mesogens 1a in the TN cell 1.
The low tilt A plates 4,4' consist, as an example, a layer of
polymerized liquid crystalline material with a tilted structure and
a low tilt angle as defined above. The tilted structure is
represented by the mesogens 4a,4a' which are oriented with their
main molecular axis tilted at an angle .theta.' relative to the
plane of the layer, wherein the tilt angle .theta.' increases in a
direction normal to the film, starting with a minimum value
.theta..sub.min ' on the side of the low tilt A plate 4,4' facing
the TN cell 1.
The dashed lines 4b and 4b' represent the projection of the
orientation directions of the mesogens 4a and 4a', respectively, in
different regions of the low tilt A plates 4,4' onto the surfaces
of the respective A plates 4,4'. The dashed lines 4b,4b' are also
identical with the principal optical axis of the respective low
tilt A plates 4,4'. In the devices shown in FIG. 2a, b, the
principal optical axis 4b,4b' of the low tilt A plate 4,4' is
oriented at right angles to the polarization direction of the
respective adjacent linear polarizer 2,2', and at right angles to
the respective adjacent orientation direction 1b,1c of the mesogens
1a in the TN cell 1.
In the devices shown in FIG. 2a, b, the mesogens at the surface of
the O plate 3,3' facing the TN cell 1 exhibit a planar orientation,
i.e. the minimum tilt angle .theta..sub.min is substantially 0
degrees. However, other values of .theta..sub.min are also
possible.
In the O plate according to the preferred embodiments as shown e.g.
in FIG. 2a, b, the minimum tilt angle .theta..sub.min is preferably
from 0 to 80.degree., in particular from 1 to 20.degree., very
preferably from 1 to 10.degree. and most preferably from 1 to
5.degree.. The maximum tilt angle .theta..sub.max in an O plate
according to these preferred embodiments is preferably from 10 to
90.degree., in particular from 20 to 90.degree., very preferably
from 30 to 90.degree., most preferably from 40 to 90.degree..
In the devices shown in FIG. 2a, b, the mesogens at the surface of
the low tilt A plate 4,4' facing the TN cell 1 exhibit a planar
orientation, i.e. the minimum tilt angle .theta..sub.min ' is
substantially 0 degrees. However, other values of .theta..sub.min '
are also possible.
In the low tilt A plate according to the preferred embodiments as
shown e.g. in FIG. 2a, b, the minimum bit angle .theta..sub.min '
is preferably from 0 to 10.degree., in particular from 0.5 to
3.degree.. The maximum tilt angle .theta..sub.max ' in an low tilt
A plate according to these preferred embodiments is preferably from
1 to 10.degree., in particular from 3 to 8.degree..
In the devices shown in FIG. 2a, b, the low tilt A plate is a film
that exhibits a tilted and splayed structure. It is, however, also
possible to use a low tilt A plate with a tilted, but not splayed
structure, wherein the tilt angle is substantially constant
throughout the film.
Apart from the preferred embodiments as depicted in FIG. 2a, b,
other combinations and stack formats of the O plates and low tilt A
plates are also possible.
For example, in the preferred devices shown in FIG. 2a, b, the O
plate 3 and the adjacent A plate 4, and/or the O plate 3' and the
adjacent low tilt A plate 4', are mutually exchangeable with each
other. Furthermore, the compensators or entire ORF stacks on one
side of the TN cell are mutually exchangeable with the compensators
or entire film stacks on the opposite side of the TN cell.
In the inventive devices exemplarily shown in FIG. 2a, b, the
optical axes 3b,3b' of the O plates 3,3' and the optical axes
4b,4b' of the low tilt A plates 4,4' are either parallel or at
right angles to the orientation direction 1b,1c of the mesogens 1a
in the TN cell 1 and to the polarization direction of the
polarizers 2,2'.
In another preferred embodiment of the present invention, the
optical axes 3b,3b' of the O plates 3,3' are twisted within the
film plane, at an angle.+-..delta., and the optical axes 4b,4b' of
the low tilt A plates 4,4' are twisted within the film plane, at an
angle.+-..delta., relative to the optical axes of the the
orientation direction 1b,1c of the mesogens 1a in the TN cell 1 and
to the polarization direction of the polarizers 2,2'. The absolute
value of said twist angle.+-..delta. is preferably from 1 to
15.degree., very preferably from 5 to 10.degree..
Further to the preferred embodiments shown in FIG. 2a, b, a
compensator according to the present invention may also comprise
more than one O plate and/or more than one A plate.
In case the inventive compensator comprises two or more O plates,
the optical axes of the O plates can be parallel one to another, or
be oriented at an angle with one another. Preferably the optical
axes of the O plates are oriented either parallel or at right
angles to each other.
In case an inventive compensator comprises two or more O plates,
each O plate can be arranged relative to the closest successive O
plate such that their respective surfaces with minimum tilt angle
.theta..sub.min are facing each other, or such that their
respective surfaces with maximum tilt angle .theta..sub.max are
facing each other, or such that the surface of a first O plate with
minimum tilt angle .theta..sub.min is facing the surface of the
closest successive O plate with maximum tilt angle
.theta..sub.max.
Further preferred arrangements of two or more O plates in an
inventive compensator are those as described in WO 98/12584, in
particular those according to the preferred embodiments described
in WO 98/12584 on pages 8-11 and in FIGS. 1a, 1b and 1c.
In another preferred embodiment of the present invention, the
optical compensator comprises one or more, especially preferably
one or two, negative C plates. As a negative C plate, it is
possible to use for example a negatively birefringent plastic
substrate on which the twisted and/or the O plate are provided.
The devices shown in FIG. 2a, b comprise splayed O plates.
Alternatively, it is possible to use tilted, but not splayed, O
plates instead of, or in addition to splayed O plates in the
inventive LC displays. Preferably, however the inventive LC
displays do comprise one or more splayed O plates.
As an O plate for the inventive compensator it is possible to use
an optical film comprising a polymerized liquid crystal material
with tilted or splayed structure, as described in the U.S. Pat. No.
5,619,352, WO 97/44409, WO 97/44702, WO 97/44703 or WO 98/12584,
with the entire disclosure of these documents being incorporated
into this application by way of reference.
As an O plate, it is also possible to use a multilayer film
comprising two or more sublayers of polymerized liquid crystal
material, with each sublayer having a tilted structure with
constant tilt angle, wherein said tilt angle increases or decreases
monotonuously from one sublayer to the next sublayer throughout the
multilayer.
In a preferred embodiment of the invention, the O plate is a tilted
or splayed optical retardation film (ORF) film as described in WO
98/12584, or a film prepared in analogy to the methods disclosed
therein. According to the WO 98/12584, an ORF with tilted or
splayed structure can be obtained by coating a layer of a
polymerizable mesogenic material onto a substrate or between two
substrates, aligning the material into a tilted or splayed
orientation, and polymerizing the material by exposure to heat or
actinic radiation.
Alternatively it is possible to use as an O plate a liquid crystal
film as described in WO 96/10770, which is prepared from a
polymerizable liquid crystal material with a smectic A or smectic C
phase and a nematic phase at higher temperatures. The polymerizable
liquid crystal material is applied in its nematic phase onto a
substrate that is, e.g., covered with an alignment layer of
obliquely deposited SiO, and lowering the temperature into smectic
C phase of the material. This leads to an increase of the tilt
angle, as the material adopts its naturally tilted smectic C
structure, which is then fixed by polymerization of the liquid
crystal material. The above preparation method and possible
variations thereof are described in detail in WO 96/10770, the
entire disclosure of which is incorporated into this application by
way of reference.
It is also possible to use as an O plate an inorganic thin film
with a tilted microstructure, which can be obtained by oblique
vapor deposition of an inorganic material, e.g. Ta.sub.2 O.sub.5,
as described in WO 96/10773.
Preferably the low tilt A plate is comprising one or more layers of
polymerized liquid crystal material with a slightly tilted
structure. Such layers can be prepared by using a high pre-tilt on
the substate surface combined with a liquid crystal material which
gives planar alignment at the air interface. Alternatively a liquid
crystal material with an optimised tilt angle can be used. The
thickness d of the O plate and the thickness d' of the low tilt A
plate is in each case independently preferably from 0.1 to 10
.mu.m, in particular from 0.2 to 5 .mu.m, most preferably from 0.3
to 3 .mu.m. For some applications, a film thickness between 2 and
15 .mu.m is also suitable.
As negative C plate, it is possible to use a stretched or
uniaxially compressed plastic film, as described e.g. in U.S. Pat.
No. 4,701,028, or an inorganic thin film obtained by physical vapor
deposition, as described e.g. in U.S. Pat. No. 5,196,953.
Particularly preferred are inventive compensators wherein the O
plate is provided on a negatively birefringent substrate which
serves as negative C plate. Further preferred are inventive
compensators wherein each of the twisted and the O plate are
provided on a negatively birefringent substrate.
As a negatively birefringent film substrate for example a
uniaxially compressed plastic film, like e.g. PET, PVA, PC,
tiacetylcellulose (TAC) or diacetylcellulose (DAC) can be used.
Especially preferred are PVA, TAC and DAC films.
In a particularly preferred embodiment, the negative C plate is a
film comprising one or more layers of anisotropic material having a
highly twisted structure, wherein the helical pitch has a value
below the visible wavelength range.
A highly twisted film, which has the structure of a twisted A plate
as defined above but with a high twist angle, exhibits a
compensation performance for liquid crystal displays that is at
least equivalent to, and in some cases even better than, the
performance of a conventional negatively birefringent C-plate
retarder. In case the helical pitch of the highly twisted A plate
is such that it shows selective reflection light of a wavelength
below the visible range, the highly twisted A plate can be used as
a negative C plate in the inventive compensator.
This is an additional benefit of the present invention, since the
state of the art negatively birefringent C-plates in most cases
either require complicated manufacturing procedures such as vapour
deposition of an inorganic thin film (as described for example in
U.S. Pat. No. 5,196,953), or they require the use of negatively
birefringent materials, which are most often less easily available
and more expensive than positively birefringent materials.
Thus, FIG. 3 depicts a compensated TN-LCD device according to a
further preferred embodiment of the invention in its off-state,
i.e. when no voltage is applied, in the E mode. The device contains
a TN cell 1 with a liquid crystal layer in the twisted nematic
state sandwiched between two transparent electrodes (which are not
shown here), and a pair of linear polarizers 2,2'. The device
further comprises on each side of the TN cell an inventive
compensator consisting of an O plate 3,3', a low tilt A plate 4,4,'
a negative C plate 5,5' and a highly twisted A plate 6,6' which
serve as negative C plate, wherein the O plates 3,3' are provided
on the negative C plates 5,5' which serve as substrates. he stack
format of the optical components in FIG. 3 corresponds to type [H]
of table 1 above.
It should be noted that FIGS. 1-3 are not intended to depict the
real proportions of the individual device components. Thus, for
example in a real device according to FIG. 3 the negative C plates
5,5' are expected to have a higher thickness than the highly
twisted A plates 6,6', i.e, different than suggested by the
dimensions shown FIG. 3.
The highly twisted A plate preferably exhibits a chiral liquid
crystal material, e.g. a cholesteric material, with a highly
twisted structure wherein the main molecular axes of the mesogens
are helically twisted at more than one full helix turn around an
axis perpendicular to the plane of the film.
The twist angle .phi. of the twisted A plate can also be expressed
by the helical pitch p of the liquid crystalline material and the
thickness d" of the A plate according to the equation
The helical pitch p of a highly twisted A plate in an inventive
compensator is preferably less than 250 nm, so that the film does
not reflect visible light. Preferably the pitch p is from 50 to 250
nm, in particular from 100 to 250 nm.
The thickness of a highly twisted A plate is preferably from 0.1 to
5 .mu.m, in particular from 0.2 to 3 .mu.m, very preferably from
0.3 to 1.5 .mu.m.
A highly twisted A plate according to this preferred embodiment
preferably comprises one or more layers of polymerized cholesteric
liquid crystal material as described for example in GB 2,315,072,
in particular as described therein on page 2-14 and in examples
1-5. These films do exhibit a very small helical pitch leading to a
reflection wavelength in the UV range. For the purposes of the
present invention, highly twisted A plates with a pitch as
described in the GB 2,315,072, most preferably with an even smaller
pitch, are preferred. These films can be prepared according to or
in analogy to the methods described in GB 2,315,072.
Alternatively, it is also possible to use as highly twisted A plate
one or more layers of platelets or platelet-shaped flakes
comprising an oriented polymerized cholesteric liquid crystal
material with planar orientation, these platelets or flakes being
dispersed in a light-transmissive binder, and being oriented such
that the helix axis of the cholesteric liquid crystal material
extends substantially perpendicular to the plane of the layer.
Suitable platelets or flakes are described e.g. in WO 97/30136
(Merck), WO 96/18129 (CRL), U.S. Pat. No. 5,364,557 (Faris), EP 0
601 483, EP 0 773 250 or U.S. Pat. No. 5,827,449 (Wacker).
In a preferred embodiment of the invention, the highly twisted A
plate is a film as described in GB 2,315,072, or a film prepared in
analogy to the methods disclosed therein, with the entire
disclosure of this document being incorporated into this
application by way of reference.
Thus, according to GB 2,315,072 a highly twisted A plate can be
obtained by coating a layer of a chiral polymerizable mesogenic
material onto a substrate or between two substrates, aligning the
material into a twisted orientation, wherein the helical twist axis
is perpendicular to the plane of the layer, and polymerizing the
material by exposure to heat or actinic radiation.
In another preferred embodiment of the present invention the
compensator additionally comprises one or more twisted A plates
with low or moderate twist, in particular with a twist angle below
360.degree.. In these twisted A plates, the twist angle .phi. is
preferably from 90.degree. to 270.degree.. As a twisted A plate, it
is possible to use e.g. a twisted nematic polymer film as described
in the EP 0 423 881 (Philips), EP 0 576 931 (Casio) or U.S. Pat.
No. 5,243,451 (Ricoh).
In case of the twisted or highly twisted A plate, it is also
possible to use a layer of a non-polymerized liquid crystal
material. For example, a nematic liquid crystal mixture can be used
that is provided between two transparent substrates and exhibits a
planar twisted orientation, wherein the twist is induced by
different orientation of the liquid crystal molecules at the
substrates, like in a standard type TN cell, or the twist is
brought about by one or more chiral dopants added to the nematic
material. Alternatively a layer of a cholesteric liquid crystal
mixture can be used.
As linear polarizer, a standard type commercially available
polarizer can be used. In a preferred embodiment of the present
invention the linear polarizer is a low contrast polarizer. In
another preferred embodiment of the present invention the linear
polarizer is a dichroic polarizer, like a dyed polarizer.
The individual optical components in the inventive compensators and
displays, such as the liquid crystal cell, the individual retarders
and the linear polarizers, can be separated or can be laminated to
other components. They can be stacked, mounted on top of each other
or be connected e.g. by means of adhesive layers.
It is also possible that stacks of two or more retarders are
prepared by coating the liquid crystalline material of an retarder
directly onto an adjacent retarder, the latter serving as
substrate.
The optical compensator and/or the display device according to the
present invention may further comprise one or more adhesive layers
provided to the individual optical components like the liquid
crystal cell, the polarizers and the different retarders.
In case the polymerized liquid crystal material in the O plate
and/or the A plate is a polymer with high adhesion, separate
adhesive layers may also be omitted. Highly adhesive polymers are
for example liquid crystal polyepoxides. Furthermore, liquid
crystal linear polymers or crosslinked polymers with low degree of
crosslinking show higher adhesion than highly crosslinked polymers.
The above highly adhesive liquid crystal polymers are therefore
preferred for specific applications, especially for those which do
not tolerate additional adhesive layers.
The inventive compensator may also comprise one or more protective
layers provided on the surface of the individual optical components
described above.
In case of the twisted and highly twisted A plate, the
polymerizable material comprises achiral polymerizable mesogenic
compounds and further comprises at least one chiral compound. The
chiral compounds can be selected from non-polymerizable chiral
compounds, like e.g. chiral dopants as used in liquid crystal
mixtures or devices, polymerizable chiral non-mesogenic or
polymerizable chiral mesogenic compounds.
In case of the O plate and the low tilt A plate, the polymerizable
material preferably consists essentially of achiral polymerizable
mesogenic compounds.
Preferably a polymerizable mesogenic material is used that
comprises at least one polymerizable mesogen having one
polymerizable functional group and at least one polymerizable
mesogen having two or more polymerizable functional groups.
In another preferred embodiment the polymerizable material
comprises polymerizable mesogenic compounds having two or more
polymerizable functional groups (di- or multireactive or di- or
multifunctional compounds). Upon polymerization of such a mixture a
three-dimensional polymer network is formed. An optical retardation
film made of such a network is self-supporting and shows a high
mechanical and thermal stability and a low temperature dependence
of its physical and optical properties.
By varying the concentration of the multifunctional mesogenic or
non mesogenic compounds the crosslink density of the polymer film
and thereby its physical and chemical properties such as the glass
transition temperature, which is also important for the temperature
dependence of the optical properties of the optical retardation
film, the thermal and mechanical stability or the solvent
resistance can be tuned easily.
The achiral and chiral polymerizable mesogenic mono-, di- or
multireactive compounds used for the instant invention can be
prepared by methods which are known per se and which are described,
for example, in standard works of organic chemistry such as, for
example, Houben-Weyl, Methoden der organischen Chemie,
Thieme-Verlag, Stuttgart. Typical examples are described for
example in WO 93/22397; EP 0 261 712; DE 19504224; DE 4408171 and
DE 4405316. The compounds disclosed in these documents, however,
are to be regarded merely as examples that do not limit the scope
of this invention.
Examples representing especially useful monoreactive chiral and
achiral polymerizable mesogenic compounds are shown in the
following list of compounds, which should, however, be taken only
as illustrative and is in no way intended to restrict, but instead
to explain the present invention: ##STR1##
Examples of useful direactive chiral and achiral polymerizable
mesogenic compounds are shown in the following list of compounds,
which should, however, be taken only as illustrative and is in no
way intended to restrict, but instead to explain the present
invention ##STR2##
In the above formulae, P is a polymerizable group, preferably an
acryl, methacryl, vinyl, vinyloxy, propenyl ether, epoxy or stytryl
group, x and y are each independently 1 to 12, A is 1,4-phenylene
that is optionally mono- di or trisubstituted by L.sup.1 or
1,4-cyclohexylene, v is 0 or 1, Z.sup.0 is --COO--, --OCO--,
--CH.sub.2 CH.sub.2 -- or a single bond, Y is a polar group,
R.sup.0 is an unpolar alkyl or alkoxy group, Ter is a terpenoid
radical like e.g. menthyl, Chol is a cholesteryl group, and L.sup.1
and L.sup.2 are each independently H, F, Cl, CN or an optionally
halogenated alkyl, alkoxy, alkylcarbonyl, alkoxycarbonyl or
alkoxycarbonyloxy group with 1 to 7 C atoms.
The term `polar group` in this connection means a group selected
from F, Cl, CN, NO.sub.2, OH, OCH.sub.3, OCN, SCN, an optionally
fluorinated carbonyl or carboxyl group with up to 4 C atoms or a
mono- oligo- or polyfluorinated alkyl or alkoxy group with 1 to 4 C
atoms.
The term `unpolar group` means an alkyl group with 1 or more,
preferably 1 to 12 C atoms or an alkoxy group with 2 or more,
preferably 2 to 12 C atoms.
In case of the prepartion of the twisted A plate, the chiral
polymerizable mesogenic material may comprise one or more
non-polymerizable chiral dopants in addition or alternatively to
chiral polymerizable mesogenic compounds. Especially preferred are
chiral dopants with a high helical twisting power (HTP), in
particular those disclosed in WO 98/00428. Further typically used
chiral dopants are e.g. the commercially available S 1011, R 811 or
CB 15 (from Merck KGaA, Darmstadt, Germany).
Especially preferred are chiral non-polymerizable dopants selected
from the following formulae ##STR3##
including the (R,S), (S,R), (R,R) and (S,S) enantiomers not shown,
wherein E and F are each independently 1,4-phenylene or
trans-1,4-cyclohexylene, v is 0 or 1, Z.sup.0 is --COO--, --OCO--,
--CH.sub.2 CH.sub.2 -- or a single bond, and R is alkyl, alkoxy or
alkanoyl with 1 to 12 C atoms.
The compounds of formula IIIa and their synthesis are described in
WO 98/00428. The compounds of formula IIIb and their synthesis are
described in GB 2,328,207.
The above chiral compounds of formula IIIa and IIIb exhibit a very
high helical twisting power (HTP), and are therefore particularly
useful for the preparation of a highly twisted ORF as used in the
present invention.
The polymerizable mesogenic material is coated onto substrate,
aligned into a uniform orientation and polymerized according to a
process as described in WO 98/12584 or GB 2,315,072, thereby
permanently fixing the orientation of the polymerizable mesogenic
material.
As a substrate for example a glass or quartz sheet or a plastic
film or sheet can be used. It is also possible to put a second
substrate on top of the coated mixture prior to and/or during
and/or after polymerization. The substrates can be removed after
polymerization or not. When using two substrates in case of curing
by actinic radiation, at least one substrate has to be transmissive
for the actinic radiation used for the polymerization. Isotropic or
birefringent substrates can be used. In case the substrate is not
removed from the polymerized film after polymerization, preferably
isotropic substrates are used.
Preferably at least one substrate is a plastic substrate such as
for example a film of polyester such as polyethyleneterephthalate
(PET), of polyvinylalcohol (PVA), polycarbonate (PC) or
triacetylcellulose (TAC), especially preferably a PET film or a TAC
film. As a birefringent substrate for example an uniaxially
stretched plastic film can be used. For example PET films are
commercially available from ICI Corp. under the trade name
Melinex.
The polymerizable mesogenic material can also be dissolved in a
solvent, preferably in an organic solvent. The solution is then
coated onto the substrate, for example by spin-coating or other
known techniques, and the solvent is evaporated off before
polymerization. In most cases it is suitable to heat the mixture in
order to facilitate the evaporation of the solvent.
For preparing an ORF with twisted structure, it is necessary to
achieve planar alignment in the layer of the chiral polymerizable
material, i.e. with the helical axis being oriented substantially
perpendicular to the plane of the layer. Planar alignment can be
achieved for example by shearing the material, e.g. by means of a
doctor blade. It is also possible to apply an alignment layer, for
example a layer of rubbed polyimide or sputtered SiO.sub.x, on top
of at least one of the substrates.
Planar alignment of the polymerizable mesogenic material can also
be achieved by directly rubbing the substrate, i.e. without
applying an additional alignment layer. This is a considerable
advantage as it allows a significant reduction of the production
costs of the optical retardation film. In this way a low tilt angle
can easily be achieved.
For example rubbing can be achieved by means of a rubbing cloth,
such as a velvet cloth, or with a flat bar coated with a rubbing
cloth. In a preferred embodiment of the present invention rubbing
is achieved by means of a at least one rubbing roller, like e.g. a
fast spinning roller that is brushing across the substrate, or by
putting the substrate between at least two rollers, wherein in each
case at least one of the rollers is optionally covered with a
rubbing cloth. In another preferred embodiment of the present
invention rubbing is achieved by wrapping the substrate at least
partially at a defined angle around a roller that is preferably
coated with a rubbing cloth.
The polymerizable composition according to the the present
invention may also comprise one or more surfactans to improve
planar alignment. Suitable surfactants are described for example in
J. Cognard, Mol. Cryst. Liq. Cryst 78, Supplement 1, 1-77 (1981).
Particularly preferred are non-ionic surfactants, such as the
commercially available fluorocarbon surfactants Fluorad 171 (from
3M Co.), or Zonyl FSN (from DuPont). Preferably the polymerizable
mixture comprises 0.01 to 5%, in particular 0.1 to 3%, very
preferably 0.2 to 2% by weight of surfactants.
The orientation of the mesogenic material depends, inter alia, on
the film thickness, the type of substrate material, and the
composition of the polymerizable mesogenic material. It is
therefore possible, by changing these parameters, to control the
structure of the ORF, in particular specific parameters such as the
tilt angle and its degree of variation.
Thus, for the preparation of the O plate, it is possible to adjust
the alignment profile in the direction perpendicular to the film
plane by appropriate selection of the ratio of monoreactive
mesogenic compounds, i.e. compounds with one polymerizable group,
and direactive mesogenic compounds, i.e. compounds with two
polymerizable groups.
For an O plate with strong splay, i.e. a large variation of the
tilt angle throughout the thickness of the film, preferably the
ratio of mono- to direactive mesogenic compounds should be in the
range of 6:1 to 1:2, preferably 3:1 to 1:1, especially preferably
about 3:2.
Another effective means to adjust the desired splay geometry is to
use a defined amount of dielectrically polar polymerizable
mesogenic compounds in the polymerizable mesogenic material. These
polar compounds can be either monoreactive or direactive. They can
be either dielectrically positive or negative. Most preferred are
dielectrically positive and monoreactive mesogenic compounds.
The amount of the polar compounds in the mixture of polymerizable
mesogenic material is preferably 1 to 80%, especially 3 to 60%, in
particular 5 to 40% by weight of the total mixture.
Polar mesogenic compound in this connection means a compound with
one or more polar groups as defined above. Especially preferred are
monoreactive polar compounds selected from formulae Ia to Ic given
above.
Furthermore, these polar compounds preferably exhibit a high
absolute value of the dielectric anisotropy .DELTA..epsilon., which
is typically higher than 1.5. Thus, dielectrically positive
compounds preferably exhibit .DELTA..epsilon.>1.5 and
dielectrically negative polar compounds preferably exhibit
.DELTA..epsilon.<-1.5. Very preferred are dielectrically
positive polar compounds with .DELTA..epsilon.>3, in particular
with .DELTA..epsilon.>5.
Polymerization of the polymerizable mesogenic material takes place
by exposing it to heat or actinic radiation. Actinic radiation
means irradiation with light, like UV light, IR light or visible
light, irradiation with X-rays or gamma rays or irradiation with
high energy particles, such as ions or electrons. Preferably
polymerization is carried out by UV irradiation.
As a source for actinic radiation for example a single UV lamp or a
set of UV lamps can be used. When using a high lamp power the
curing time can be reduced. Another possible source for actinic
radiation is a laser, like e.g. a UV laser, an IR laser or a
visible laser.
The polymerization is carried out in the presence of an initiator
absorbing at the wavelength of the actinic radiation. For example,
when polymerizing by means of UV light, a photoinitiator can be
used that decomposes under UV irradiation to produce free radicals
or ions that start the polymerization reaction.
When curing polymerizable mesogens with acrylate or methacrylate
groups, preferably a radical photoinitiator is used, when curing
polymerizable mesogens vinyl and epoxide groups, preferably a
cationic photoinitiator is used.
It is also possible to use a polymerization initiator that
decomposes when heated to produce free radicals or ions that start
the polymerization.
As a photoinitiator for radical polymerization for example the
commercially available Irgacure 651, Irgacure 184, Darocure 1173 or
Darocure 4205 (all from Ciba Geigy AG) can be used, whereas in case
of cationic photopolymerization the commercially available UVI 6974
(Union Carbide) can be used.
The polymerizable mesogenic material preferably comprises 0.01 to
10%, very preferably 0.05 to 5%, in particular 0.1 to 3% of a
polymerization initiator. UV photoinitiators are preferred, in
particular radicalic UV photoinitiators.
The curing time is dependening, inter alia, on the reactivity of
the polymerizable mesogenic material, the thickness of the coated
layer, the type of polymerization initiator and the power of the UV
lamp. The curing time according to the invention is preferably not
longer than 10 minutes, particularly preferably not longer than 5
minutes and very particularly preferably shorter than 2 minutes.
For mass production short curing times of 3 minutes or less, very
preferably of 1 minute or less, in particular of 30 seconds or
less, are preferred.
In addition to polymerization initiators the polymerizable material
may also comprise one or more other suitable components such as,
for example, catalysts, stabilizers, chain-transfer agents,
co-reacting monomers or surface-active compounds. In particular the
addition of stabilizers is preferred in order to prevent undesired
spontaneous polymerization of the polymerizable material for
example during storage.
As stabilizers in principal all compounds can be used that are
known to the skilled in the art for this purpose. These compounds
are commercially available in a broad variety. Typical examples for
stabilizers are 4-ethoxyphenol or butylated hydroxytoluene
(BHT).
Other additives, like e.g. chain transfer agents, can also be added
to the polymerizable material in order to modify the physical
properties of the inventive polymer film. When adding a chain
transfer agent, such as monofunctional thiol compounds like e.g.
dodecane thiol or multifunctional thiol compounds like e.g.
trimethylpropane tri(3-mercaptopropionate), to the polymerizable
material, the length of the free polymer chains and/or the length
of the polymer chains between two crosslinks in the inventive
polymer film can be controlled. When the amount of the chain
transfer agent is increased, the polymer chain length in the
obtained polymer film is decreasing.
It is also possible, in order to increase crosslinking of the
polymers, to add up to 20% of a non mesogenic compound with two or
more polymerizable functional groups to the polymerizable material
alternatively or in addition to the di- or multifunctional
polymerizable mesogenic compounds to increase crosslinking of the
polymer.
Typical examples for difunctional non mesogenic monomers are
alkyldiacrylates or alkyldimethacrylates with alkyl groups of 1 to
20 C atoms. Typical examples for non mesogenic monomers with more
than two polymerizable groups are trimethylpropanetrimethacrylate
or pentaerythritoltetraacrylate.
In another preferred embodiment the mixture of polymerizable
material comprises up to 70%, preferably 3 to 50% of a non
mesogenic compound with one polymerizable functional group. Typical
examples for monofunctonal non mesogenic monomers are
alkylacrylates or alkylmethacrylates.
It is also possible to add, for example, a quantity of up to 20% by
weight of a non polymerizable liquid-crystalline compound to adapt
the optical properties of the optical retardation film.
In some cases it is of advantage to apply a second substrate to aid
alignment and exclude oxygen that may inhibit the polymerization.
Alternatively the curing can be carried out under an atmosphere of
inert gas. However, curing in air is also possible using suitable
photoinitiators and high UV lamp power. When using a cationic
photoinitiator oxygen exclusion most often is not needed, but water
should be excluded. In a preferred embodiment of the invention the
polymerization of the polymerizable mesogenic material is carried
out under an atmosphere of inert gas, preferably under a nitrogen
atmosphere.
To obtain a polymer film with the desired molecular orientation the
polymerization has to be carried out in the liquid crystal phase of
the polymerizable mesogenic material. Therefore, preferably
polymerizable mesogenic compounds or mixtures with low melting
points and broad liquid crystal phase ranges are used. The use of
such materials allows to reduce the polymerization temperature,
which makes the polymerization process easier and is a considerable
advantage especially for mass production.
The selection of suitable polymerization temperatures depends
mainly on the clearing point of the polymerizable material and
inter alia on the softening point of the substrate. Preferably the
polymerization temperature is at least 30 degrees below the
clearing temperature of the polymerizable mesogenic mixture.
Polymerization temperatures below 120.degree. C. are preferred.
Especially preferred are temperatures below 90.degree. C., in
particular temperatures of 60.degree. C. or less.
The invention is further explained by the following examples.
Therein, the following abbreviations are used:
.theta. tilt angle [degrees] .phi. twist angle [degrees] p helical
pitch [nm] n.sub.e extraordinary refractive index (at 20.degree. C.
and 589 nm) n.sub.o ordinary refractive index (at 20.degree. C. and
589 nm) .epsilon..sub..parallel. dielectric constant parallel to
the long molecular axis (at 20.degree. C. and 1 kHz)
.epsilon..sub..perp. dielectric constant perpendicular to the long
molecular axis (at 20.degree. C. and 1 kHz) K.sub.11 first elastic
constant K.sub.22 second elastic constant K.sub.33 third elastic
constant V.sub.on threshold voltage [V] V.sub.off saturation
voltage [V] d layer thickness [.mu.m]
COMPARISON EXAMPLE A
An uncompensated standard type TN-LCD device of the E mode as
depicted in FIG. 1a, comprising a TN cell 1 and a pair of linear
polarizers 2,2' has the following parameters
n.sub.e 1.5700 n.sub.o 1.4785 .epsilon..sub..perp. 3.5
.epsilon..sub..parallel. 10.8 K.sub.11 11.7 K.sub.22 5.7 K.sub.33
15.7 d 5.25 .mu.m pre-tilt 2.degree. V.sub.on 4.07 V V.sub.off 1.56
V
FIG. 4a depicts the isocontrast plot of the display, showing ranges
of identical contrast in steps of 10%. The isocontrast plots are
measured as luminance at V.sub.on /luminance at V.sub.off.
FIGS. 4b and 4c show 8 grey levels (given as transmission versus
viewing angle), on a linear luminance scale in horizontal and
vertical viewing planes, respectively. Ideally, the grey level
lines should be parallel, where they cross, grey level inversion
occurs. The latter is a serious disadvantage especially for the
darker grey levels. It can be seen in FIG. 4b that levels 7 and 8
are very poor even at low angles such as 30.degree. in horizontal
direction, and in FIG. 4c that the levels cross at angles of
30.degree. and higher in vertical direction.
The polarisers can be any standard polariser used in normal LCD
displays.
V.sub.on, V.sub.off correspond to values generally adopted in TN
and STN-LCD displays.
COMPARISON EXAMPLE B
An uncompensated standard type TN-LCD device of the O mode as
depicted in FIG. 1b, comprising a TN cell 1 and a pair of linear
polarizers 2,2', has the parameters as given in comparison example
A.
FIG. 5a depicts the isocontrast plot of the display, showing ranges
of identical contrast in steps of 10%. The isocontrast plots are
measured as luminance at V.sub.on /luminance at V.sub.off.
FIGS. 5b and 5c show the grey levels in horizontal and vertical
viewing planes, respectively. It can be seen in FIG. 5b that levels
7 and 8 are very poor even at low angles such as 30.degree. in
horizontal direction, and in FIG. 5c that the levels cross at
angles of 30.degree. and higher in vertical direction.
EXAMPLE 1
A compensated TN-LCD device of the O mode according to the present
invention as depicted in FIG. 2a consists of a TN cell 1 with a
liquid crystal layer in a twisted nematic state, a pair of linear
polarizers 2,2', two splayed O plates 3,3', two low tilt A plates
4,4', and four negative C plates 5,5' serving as substrates for the
O plates and low tilt A plates. The stack format of the optical
components corresponds to type [H] of table 1 above.
The TN cell 1 and the polarizers 2,2' are as defined in comparison
example A.
The O plates 3,3' exhibit a splayed structure with the tilt angle
.theta. ranging from .theta..sub.min on one surface to
.theta..sub.max on the opposite surface.
The parameters of the O plates 3,3' are as follows
.theta..sub.min 2.degree. .theta..sub.max 88.degree.
.theta..sub.ave 45.degree. n.sub.e 1.610 n.sub.o 1.495 d 1.2 .mu.m
retardation 70 nm
The parameters of the low tilt A plates 4,4' are as follows
.theta..sub.min 0.degree. .theta..sub.max 8.degree. .theta..sub.ave
4.degree. n.sub.e 1.610 n.sub.o 1.495 d' 0.91 .mu.m retardation 104
nm
In the display device according to example 1, the orientation
directions of the optical axes of the individual optical films
within the film plane are given in table 2 below. For a better
understanding, the orientation directions of 0.degree., 90.degree.,
180.degree. and 270.degree. are also depicted by the arrows on the
left side of FIGS. 2a, 2b and 3.
TABLE 2 Orientation direction of the optical axes of individual
components in a display according to example 1 left polarizer 2
45.degree. O plate 3 225.degree. low tilt A plate 4 135.degree. low
tilt A plate 4' 225.degree. O plate 3' 135.degree. right polarizer
2' 315.degree.
FIG. 6a shows the isocontrast plot of the display, FIGS. 6b and 6c
show the grey levels (transmission vesus viewing angle) in
horizontal and vertical directions respectively.
In the isocontrast plot FIG. 6a it can be seen that the display has
a viewing angle that is significantly larger in horizontal
direction, compared to the uncompensated display of example B, and
is also slightly improved in vertical direction. In FIGS. 6b and 6c
it can be seen that the grey levels 7 and 8 in horizontal direction
are significantly improved compared to the uncompensated display of
example B, and are also improved in vertical direction at negative
angles.
EXAMPLE 2
A compensated TN-LCD device of the E mode according to the present
invention comprises the individual components and the stack format
as shown in FIG. 2b.
The display according to example 2 relates to a preferred
embodiment of the present invention, wherein the O plates 3,3' and
the low tilt A plates 4,4' exhibit the same retardation.
In addition, the devices according to example 2 relate to a
preferred embodiment, wherein the optical axes 3b,4b of the O plate
3 and the low tilt A plate 4 are twisted clockwise, at an
angle+.delta., and the optical axes 3b',4b' of the O plate 3' and
the low bit A plate 4' are twisted counterclockwise, at an
angle-.delta., in a plane parallel to the film planes and relative
to the optical axes of the other optical elements in the
display.
In the device according to example 2 the orientation directions of
the optical axes of the polarizers 2,2' are as given in table 3
below, whereas the optical axes 3b and 4b of the O plate 3 and low
tilt A plate 4 are oriented at 219.degree. and 129.degree.
respectively, and the optical axes 3b' and 4b' of the O plate 3'
and low tilt A plate 4' are oriented at 231.degree. and
141.degree., respectively. Thus, the angle .delta. in example is
.+-.6
The thickness of the O plates is 1.427 .mu.m.
The thickness of the low tilt A plates is 0.711 .mu.m.
The retardation of the O plates and low tilt A plates is 82 nm.
The other parameters are as given in example 1.
The orientations of the individual components are as given in table
3.
TABLE 3 Orientation direction of the optical axes of individual
components in a display according to example 2 left polarizer 2
315.degree. O plate 3 219.degree. low tilt A plate 4 129.degree.
low tilt A plate 4' 231.degree. O plate 3' 141.degree. right
polarizer 2' 45.degree.
FIG. 7a shows the isocontrast plot of the display, FIG. 7b and 7c
show the grey levels in horizontal and vertical directions
respectively. It can be seen that, compared to the uncompensated
display of example A, the viewing angle is significantly enlarged.
The 100-1 isocontrast area is +/-50.degree. in the horizontal
direction and +60 to -30.degree.
FIG. 8a shows the isocontrast plot of the display, FIGS. 8b and 8c
show the grey levels in horizontal and vertical directions
respectively. It can be seen that, compared to the uncompensated
display of example B, the viewing angle is significantly enlarged.
The 100-1 isocontrast area is +/-45.degree. in the horizontal
direction and +55.degree./-25.degree. in the vertical direction.
The 10-1 isocontrast area is +/-60.degree. in the horizontal
direction and +60/-50.degree. in the vertical direction. The grey
levels are improved both in horizontal and vertical direction.
The preceding examples can be repeated with similar success by
substituting the generically or specifically described reactants
and/or operating conditions of this invention for those used in the
preceding examples.
From the foregoing description, one skilled in the art can easily
ascertain the essential characteristics of this invention, and
without departing from the spirit and scope thereof, can make
various changes and modifications of the invention to adapt it to
various conditions and usages. in the vertical direction. The 10-1
isocontrast area is +/-60.degree. in the horizontal direction and
+60/-50.degree. in the vertical direction. The grey levels are
improved both in horizontal and vertical direction.
EXAMPLE 3
A compensated TN-LCD device of the E mode according to the present
invention as depicted in FIG. 3 consists of a TN cell 1 with a
liquid crystal layer in a twisted nematic state, a pair of linear
polarizers 2,2', two splayed O plates 3,3', two low tilt A plates
4,4', two negative C plates 5,5' serving as substrates for the O
plates, and two highly twisted A plates 6,6' that have the optical
performance of a negative C plate and are situated between O plate
3 and low tilt A plate 4, and between O plate 3' and low tilt A
plate 4', respectively. The stack format of the optical components
corresponds to type [H] of table 1 above.
In the device according to example 3, the optical axes of the O
plates 3,3' and the A plates 4,4' are twisted relative to the other
optical components at an angle .delta. of .+-.6.degree. as defined
above. The orientation of the other components is as given in table
3, example 2.
The parameters of the highly twisted A plates 6,6' are as
follows
n.sub.e 1.610 n.sub.o 1.495 d" 0.890 .mu.m pitch p 200 nm
The average tilt angle .theta..sub.ave of the O plates is
45.degree..
The thickness of the O plates is 1.427 .mu.m.
The thickness of the low tilt A plates is 0.711 .mu.m.
The retardation of the O plates and low tilt A plates is 82 nm.
The other parameters are as given in example 1.
* * * * *